Ultraviolet transmitting glasses for EPROM windows

ABSTRACT

Ultraviolet transmitting glasses exhibiting coefficients of thermal expansion between 46-52×10 -7  /°C. and transmittances at a thickness of 1 mm of at least 80% at a wavelength of 254 nm which are essentially free of fluoride and consist essentially, in mole percent, of 60-70% SiO 2 , 16-20% B 2  O 3 , 1-8% Al 2  O 3 , 1-6% Li 2  O, 2.5-5% N 2  O, 0-3% K 2  O, and 0-1.25% Cl, w9herein the mole ratio R 2  O:R 2  O 3  is greater than 0.3, but less than 0.5.

BACKGROUND OF THE INVENTION

This is a division of application Ser. No. 316,184 filed Feb. 27, 1989.

In general terms, an Erasable, Programmable, Read-Only, Memory (EPROM)package comprises:

(a) an insulating board having a chip mounting portion and havingconductive interconnecting patterns thereon; one end of each of thosepatterns being adjacent to the chip mounting portion;

(b) an EPROM chip mounted on the chip mounting portion; the chip havinga plurality of electrodes on a surface thereof;

(c) means for connecting said electrodes to the one end of each of thepatterns; and

(d) a covering insulating board containing a window composed of anultraviolet transmitting glass.

This invention is directed to the production of a family of ultraviolettransmitting glass compositions expressly designed for use as windows inEPROM chips. The following four criteria have been determined to bequite desirable in glasses designed for that application:

First, the glasses must manifest coefficients of thermal expansion(0°-300° C.) between 46-52×10⁻⁷ /°C. to insure sound sealing with Al₂ O₃which is commonly used as the insulating material in EPROM packages;

Second, the glasses must demonstrate transmittances of at least 80% inthicknesses of 1 mm at a wavelength of 254 nm;

Third, the gasses must possess softening points below 700° C. to permitrapid sealing to Al₂ O₃ ;

Fourth, the glass compositions must be free of fluoride inasmuch asvolatilization of fluorine during melting leads to difficulties in glassforming and is an environmental hazard.

The types of aluminas used by EPROM fabricators vary widely. Hence, thevarious aluminas have different microstructures and, as a result,different intrinsic strengths and thermal expansions. As anillustration, the coefficient of thermal expansion ofcommercially-marketed aluminas (0°-300° C.) can vary over the range ofabout 65-80×10⁻⁷ /°C. Such substantial variation quite obviously affectsboth the integrity of a glass-alumina seal and the amount of stressgenerated in the alumina during cooling of the seal, given any singleglass composition window.

In common sealing practice, an expansion mismatch between the componentsbeing sealed together of less than 500 parts per million (ppm) isgenerally interpreted as a good seal, while differentials of less than1000 ppm can be resolved into sound seals in more extraordinarycircumstances. In the fabrication of EPROM packages, experiencerepeatedly demonstrated that expansion differentials between the glassand alumina greater than about 500 ppm create excessive stresses on bothcomponents of a seal. In such seals the excessive stress developed inthe alumina has been deemed to cause the alumina components of the EPROMpackage to fracture.

Another extremely significant circumstance which must be considered indesigning materials for EPROM packages is the fact that such packagesmust survive very demanding thermal shock requirements. Hence, thehigher thermal conductivity of alumina during heating, when compared toconventional glasses, causes it to expand at a more rapid rate than theglass, thereby hazarding a loss of hermeticity in a seal with glass.Accordingly, a thermal expansion differential between the glass andalumina no less than about 200 ppm has been considered essential.

Therefore, the primary objective of the present invention was to developglasses suitable for fabrication as windows for EPROM chips whichsatisfy the four abovementioned criteria and, in addition, exhibit athermal expansion differential between about 200-500 ppm when sealed inalumina.

SUMMARY OF THE INVENTION

I have found that objective can be achieved in glass compositionsessentially free of fluoride and consisting essentially, expressed interms of mole percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                              60-70         Na.sub.2 O                                                                            2.5-5                                          B.sub.2 O.sub.3                                                                        16-20         K.sub.2 O                                                                             0-3                                            Al.sub.2 O.sub.3                                                                       1-8           Li.sub.2 O                                                                            1-6                                            ______________________________________                                    

but wherein the mole ratio R₂ O (Li₂ O+Na₂ O+K₂ O):R₂ O₃ (B₂ O₃ +Al₂ O₃)is greater than 0.3, but less than 0.5. Observance of this ratio iscritical in securing glasses exhibiting the demanded ultraviolettransmittance, while maintaining the desired low softening point andcoefficient of thermal expansion. Hence, where the level of alkali metaloxide is too low, the glass becomes hard; at too high levels of alkalimetal oxides, the ultraviolet transmission is adversely affected.Whereas exact conversion of mole percent to weight percent is notmathematically possible, the above ranges in mole percent closelyapproximate the following composition intervals expressed in terms ofweight percent:

    ______________________________________                                        SiO.sub.2                                                                              61-69         Li.sub.2 O                                                                            0.5-3                                          B.sub.2 O.sub.3                                                                        18-22         Na.sub.2 O                                                                              3-5.25                                       Al.sub.2 O.sub.3                                                                       1.5-12        K.sub.2 O                                                                             0-3                                            ______________________________________                                    

Up to about 1.25 mole percent (≈1 weight percent) chloride may beincluded in its customary role as a fining agent for borosilicateglasses. Also, minor amounts (most preferably totalling less than 3 molepercent) of compatible metal oxides may be included to modify themelting and/or forming character of the glasses, or to impart somedesired physical property thereto. Nevertheless, such additions must notcause a significant adverse impact upon the four above-describedcriteria. For example, CaO in an amount of about 0.3 weight percentappears to improve the melting character of the glasses.

PRIOR ART

My U.S. Pat. No. 4,792,535 was also directed to the production of alkalimetal boroaluminosilicate glasses exhibiting in 1 mm thickness atransmittance of ultraviolet radiation at a wavelength of 254 nm of atleast 80%. The glasses demonstrated coefficients of thermal expansion(0°-300° C.) between 56-62×10⁻⁷ /°C., i.e., higher than those of thepresent inventive glasses, and consisted essentially, in weight percent,of

    ______________________________________                                        SiO.sub.2                                                                              58-62         Li.sub.2 O                                                                              1-2.5                                        B.sub.2 O.sub.3                                                                        15-18         Na.sub.2 O                                                                            5.5-6.5                                        Al.sub.2 O.sub.3                                                                       11.5-14.5     K.sub.2 O                                                                             0-2                                            ______________________________________                                    

Those compositions are not only outside the range of Na₂ O demanded inthe glasses of the present invention and the ranges of SiO₂ and B₂ O₃generally lower and that of Al₂ O₃ generally higher, but also, and mostvitally, there is not even an allusion to the need for maintaining theR₂ O and R₂ O₃ contents within the critical ratio 0.3<R₂ O:R₂ O₃ <0.5.

U.S. Pat. No. 4,792,535 refers to a number of prior art references andother patents were cited during the prosecution of the patentapplication. I believe, however, that the patent constitutes the mostpertinent disclosure to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Table I records a group of glass compositions, expressed in terms ofmole percent on the oxide basis as calculated from the batch,illustrating the parameters of the instant invention. Table IA reportsthe same group of glasses, but expressed in terms of parts by weight onthe oxide basis. Because the sum of the individual ingredients totals orclosely approximates 100, for all practical purposes the values listedin Table IA for the several components can be considered to representweight percent. Inasmuch as it is not known with which cation(s) theremainder of the chloride fining agent is combined, it is simplytabulated as chloride in accordance with conventional glass analysispractice. The actual batch ingredients can comprise any materials,either oxides or other compounds, which, when melted together, will beconverted into the desired oxide in the proper proportions. For example,Li₂ CO₃ and Na₂ CO₃ may be used as sources for Li₂ O and Na₂ O,respectively. Nevertheless, because the presence of such impurities asiron oxide and titania deleteriously affects the ultraviolettransmission of the glass, batch materials of high purity are greatlypreferred.

The presence of iron in the ferric state (Fe⁺³) exerts an extremelydeleterious effect upon ultraviolet transmission. Consequently, eventhough highly pure batch ingredients were employed, between about 0.3-2%by weight cornstarch was included in each glass batch to assure completereduction of ferric iron to the ferrous state (Fe⁺²). The batches forthe several glasses were compounded, the ingredients carefully mixedtogether to aid in securing a homogeneous melt, and then charged intosilica crucibles. The crucibles were introduced into a furnace operatingat about 1550° C. and the batches were melted for 4-6 hours with a flowof nitrogen gas being passed over the crucibles to retain a reducingenvironment therein. Thereafter, the crucibles were poured into steelmolds to produce glass slabs having dimensions of about 5"×5"×0.5" andthose slabs were immediately transferred to an annealer operating atabout 500° C. Samples were cut from the annealed slabs and tested forultraviolet transmittance and general physical properties.

It will be appreciated that, whereas the above described procedurereflects laboratory melting and forming only, the glasses of the presentinvention can be melted in conventional glass melting facilities,including large continuously-melting tanks, and can be formed intodesired configurations utilizing techniques conventional in theglassmaking art.

Table I recites the mole ratio R₂ O:R₂ O₃, the softening point,annealing point, and strain point in °C., the coefficient of thermalexpansion (Coef. Exp.) over the range 0°-300° C. in terms of ×10⁻⁷ /°C.,the liquidus (°C.), and the % transmittance at 254 nm in a ground andpolished thickness of 1 mm.

                  TABLE I                                                         ______________________________________                                        (Mole %)                                                                      ______________________________________                                                 1       2      3     4    5     6    7                               ______________________________________                                        SiO.sub.2                                                                              68.1    71.0   66.1  65.0 65.1  69.0 64.2                            B.sub.2 O.sub.3                                                                        18.9    18.9   18.9  17.0 18.9  18.9 19.1                            Al.sub.2 O.sub.3                                                                       4.3     1.3    4.2   7.2  7.3   4.3  4.3                             Li.sub.2 O                                                                             1.3     1.5    3.4   5.5  1.3   1.5  5.4                             Na.sub.2 O                                                                             4.8     4.8    4.8   3.8  4.8   4.8  4.8                             K.sub.2 O                                                                              1.5     1.5    1.5   0.5  1.5   0.5  1.5                             Cl       1.1     1.1    1.1   1.1  1.1   1.1  0.6                             R.sub.2 O:R.sub.2 O.sub.3                                                              0.329   0.377  0.415 0.367                                                                              0.293 0.286                                                                              0.5                             Soft.    692     699    674   683  751   706  653                             Ann.     501     533    503   494  504   501  500                             Strain.  463     496    470   459  458   462  471                             Coef. Exp.                                                                             49      47     51    52   53    47   57.3                            Liquidus 791     --     --    784  1033  792  842                             Trans.   87      85     85    83   87    85   78                              ______________________________________                                                 8       9       10    11    12    13                                 ______________________________________                                        SiO.sub.2                                                                              64.1    66.1    65.0  66.2  62.2  67.8                               B.sub.2 O.sub.3                                                                        19.0    15.7    18.9  19.1  19.1  19.4                               Al.sub.2 O.sub.3                                                                       4.3     7.3     7.3   4.3   10.3  4.l                                Li.sub.2 O                                                                             1.3     4.5     1.3   1.4   1.4   --                                 Na.sub.2 O                                                                             4.8     5.2     5.7   4.8   4.8   6.5                                K.sub.2 O                                                                              5.5     --      0.6   3.5   1.5   1.2                                Cl       1.1     1.1     1.1   0.6   0.6   1.1                                Soft.    690     708     751   695   784   712                                Ann.     518     508     499   509   --    509                                Strain.  482     469     454   473   --    469                                Coef. Exp.                                                                             62      52      53    55.6  52.6  52.5                               Liquidus --      924     --    --    --    --                                 Trans.   82      75      77    83    --    84                                 ______________________________________                                    

                  TABLE IA                                                        ______________________________________                                        (Parts By Weight)                                                             ______________________________________                                                1      2      3      4    5      6    7                               ______________________________________                                        SiO.sub.2                                                                             64.2   68.4   62.9   61.8 60.2   65.5 61.6                            B.sub.2 O.sub.3                                                                       20.8   21.2   21.0   18.8 20.4   20.9 21.2                            Al.sub.2 O.sub.3                                                                      6.9    2.1    6.9    11.7 11.4   6.9  7.0                             Li.sub.2 O                                                                            0.6    0.7    1.6    2.6  0.6    0.7  2.6                             Na.sub.2 O                                                                            4.7    4.8    4.7    3.7  4.6    4.7  4.8                             K.sub.2 O                                                                             2.2    2.3    2.2    0.7  2.2    0.7  2.3                             Cl      0.6    0.6    0.6    0.6  0.6    0.6  0.6                             ______________________________________                                                 8      9        10   11     12   13                                  ______________________________________                                        SiO.sub.2                                                                              59.1   62.7     60.4 61.6   56.3 63.4                                B.sub.2 O.sub.3                                                                        20.4   17.4     20.5 20.6   20.0 21.0                                Al.sub.2 O.sub.3                                                                       6.7    12.0     11.5 6.9    15.8 6.5                                 Li.sub.2 O                                                                             0.6    2.1      0.6  0.6    0.6  --                                  Na.sub.2 O                                                                             4.6    5.1      5.5  4.6    4.5  6.3                                 K.sub.2 O                                                                              7.9    --       0.9  5.1    2.1  1.8                                 Cl       0.6    0.6      0.6  0.6    0.6  1.1                                 ______________________________________                                    

An examination of Table I points out the extreme criticality ofcomposition control needed in order to produce glasses exhibiting thedemanded optical and physical properties. Hence, only Examples 1-4demonstrate the required matrix of properties. Examples 5-7 illustratethe need for maintaining the mole ratio R₂ O:R₂ O₃ greater than 0.3. butless than 0.5. Thus, the individual components of those Examples fallwithin the defined ranges of those components, but the R₂ O:R₂ O₃ ratiois either too high or two low. Examples 8-13 are glasses havingcompositions close to the ranges required in the present invention, butone or more of the individual constituents is outside.

Example 1 is deemed to be the most preferred composition.

I claim:
 1. An EPROM package comprising:(a) an insulating board having achip mounting portion and having conductive interconnecting patternsthereon; one end of each of said patterns being adjacent to said chipmounting portion; (b) an EPROM chip mounted on said chip mountingportion; said chip having a plurality of electrodes on a surfacethereof; (c) means for connecting said electrodes to one end of each ofsaid patterns; and (d) a covering insulating board containing a windowcomposed of an ultraviolet transmitting glass; in which said ultraviolettransmitting glass has a composition which is free of fluoride andconsists essentially, expressed in terms of mole percent on the oxidebasis, of

    ______________________________________                                        SiO.sub.2                                                                              60-70         Na.sub.2 O                                                                            2.5-5                                          B.sub.2 O.sub.3                                                                        16-20         K.sub.2 O                                                                             0-3                                            Al.sub.2 O.sub.3                                                                       1-8           Li.sub.2 O                                                                            1-6                                            ______________________________________                                    

Wherein the mole ratio R₂ O:R₂ O₃ is greater than 0.3, but less than0.5.
 2. An EPROM package according to claim 1 wherein the batch for saidultraviolet transmitting glass composition also contains up to 1.25% Cl.3. An EPROM package according to claim 2 wherein said ultraviolettransmitting glass has the approximate composition, as calculated fromthe batch, of

    ______________________________________                                        SiO.sub.2 68.1          Na.sub.2 O                                                                            4.8                                           B.sub.2 O.sub.3                                                                         18.9          K.sub.2 O                                                                             1.5                                           Al.sub.2 O.sub.3                                                                         4.3          Cl      1.1                                           Li.sub.2 O                                                                               1.3                                                                ______________________________________                                    